In order to pull or hold a cable under tension, a gripper device may be attached to the cable. There are many examples of this, for instance in pulling a cable through a duct, anchoring a cable end and lifting (or lowering) a cable end from ground level to a tower or from the seabed to a ship.
Lightweight submarine telecommunications cable has a fibre optic centre core typically surrounded by a rope of high tensile steel to provide axial strength, which in turn has a copper sheath, surrounded by a low tensile strength protective polyethylene outer sheath. In some examples of such cable (known as ‘lightweight protected’ or ‘lightweight screened’) an additional metallic barrier (typically an aluminium tape) and extruded outer polyethylene sheath is added to the basic lightweight cable construction to further protect the cable.
It has been found with one known gripper device that when the gripper device contacts the outer surface of such submarine telecommunications cable, there is a problem of insufficient friction between the cable layers being acted on by the gripper, and/or insufficient adhesion between the cable layers along the cable from the gripper to the free end of the cable, and consequently there is a tendency for the protective outer sheath to be stripped from the core with the result that the engaged end is lost. This is known as filleting as described in GBP 1,492,988. The gripping device in that disclosure transmits the tension necessary in the gripping operation to the cable strength portion by belaying a bight of cable about an axis.
However, it is not always possible or practicable to create a bight of cable, for instance if it is required to grip the cable very close to the end of the cable or if the tension in the cable is high and there is insufficient slack or energy available to create the bight, such as may occur on the seabed. In GBP 1,492,988 gripping is carried out using a complex arrangement including an hydraulic circuit which causes an axially mounted spool to rotate and belay the bight of cable by wrapping the cable on the spool and another hydraulic circuit to sever the cable by means of a separate clamp and blade, each movable against different shoulders on the gripper device.
An alternative known solution to grip cables with polymer sheaths, is to distribute the grip over a long length of the outer sheath by the use of a cable ‘stopper’, as described in GB 2,208,912A, hence utilising the available adhesion and friction between layers in the cable over a longer contact length. However, this is not always practicable, due to the long length of the stopper required and the manual intervention required in fitting. One case where this would not be practicable is where the gripper device needs to be remotely applied to the cable. One example of this is where the cable is under a high tension, such that it may be a risk to personnel to have to handle the cable to fit a stopper. Another example is in subsea applications, such as within a grapnel for recovering a cable end from the seabed, where both the length of a stopper, and the difficulty of remotely fitting it onto the cable without manual intervention, makes this approach impractical.
Undersea cables need to be recovered from the seabed to a ship for repair or reconfiguration, and this type of operation may need to be carried out in any water depth down to full ocean depth (up to 9000 m). In deep water the cable cannot be recovered to the ship without first cutting the cable. This is because the cable has generally been laid with insufficient slack to allow for the required increases in catenary lengths without exceeding the maximum tensile strength of the cable. Accessing deep sea cable is difficult in itself without the additional problems of remotely locating the cable, cutting and retaining the cable.
It is conventional practice to undertake a set of three grapnel drives, across the cable on the seabed. The first drive is to cut the cable, the second is to recover one end of the cable, and the third is to recover the remaining end of the cable. For the first drive, the grapnel is fitted with a blade capable of cutting the cable. For the second and third drives the grapnel is configured to capture the cable. The requirement for three separate operations is time-consuming, and the deeper the water, the longer the launch and recovery process for each operation takes. Additionally, each of the three operations has to locate the cable anew. If the grapnel passes over the cable, rather than capturing it, at least one additional attempt (a pass) will be required, and again the deeper the water, the longer this will take.
In recognition of this, several designs of grapnel have been developed in the past which combine cutting and holding functions, in which both ends of the cable can be retrieved in two rather than three grapnel drives. However such grapnels have tended to be complex and unwieldy and consequently are not widely used.
A Remotely Operated Vehicle (ROV) may also be used in place of a grapnel to cut and retrieve the cable ends. Existing grippers deployed by a manipulator arm on such ROVs use a gripper design which holds onto the external sheath of the cable only. These grippers are then intrinsically unable to hold tensions in a sheathed cable above the tension level where as above filleting is initiated.
It is an object of this invention to overcome drawbacks mentioned above.